Magnetoplumbite-type hexagonal ferrites typified by SrO.6Fe2O3, that is, M-type ferrites have hitherto been dominantly used for ferrite sintered magnets. As for such M-type ferrite magnets, efforts have been made to achieve high performance thereof, focusing on that the ferrite grain sizes are made closer to the single domain grain sizes, that ferrite grains are aligned in a magnetic anisotropic direction, and that high densities are achieved in the magnets concerned. As a result of such efforts, the properties of M-type ferrite magnets have approached the upper limits thereof, so that it is difficult to expect drastic improvements of the magnetic properties under the present circumstances.
The W-type ferrite magnet has been known as a ferrite magnet that has potentiality to surpass in magnetic properties the M-type ferrite magnet. The W-type ferrite magnet is higher by about 10% insaturationmagnetization (4πIs) than the M-type ferrite magnet and is of the same order in anisotropic magnetic field as the M-type ferrite magnet. Patent Document 1 (National Publication of International Patent Application No. 2000-501893) discloses a W-type ferrite magnet having a composition represented by SrO.2(FeO).n(Fe2O3), wherein n falls within a range from 7.2 to 7.7, and a sintered body thereof has a mean grain size of 2 μm or less and a (BH)max of 5 MGOe or more.
Patent Document 2 (Japanese Patent Laid-Open No. 11-251127) discloses a W-type ferrite magnet which has a maximum energy product surpassing the maximum energy products of the conventional M-type ferrites and has a composition different from the compositions of the conventional W-type ferrite magnets, wherein the fundamental composition thereof is represented by a formula MO.xFeO.(y−x/2)Fe2O3 (M being one or more of Ba, Sr, Pb and La) with the proviso that 1.7≦x≦2.1 and 8.8≦y≦9.3 in terms of atomic ratios. A magnet disclosed in Patent Document 2 having a composition of SrO.2FeO.8Fe2O3 has properties such that the residual magnetic flux density (Br) is 4800 G, the coercive force (iHc) is 3000 Oe, the maximum energy product ((BH)max) is 5.5 MGOe.
[Patent Document 1]: National Publication of International Patent Application No. 2000-501893
[Patent Document 2]: Japanese Patent Laid-Open No. 11-251127